The erythropoietin receptor (EpoR) has been known for its role in the proliferation and differentiation of erythroid cells. This lab has reported the detection of EPO-R mRNA in placenta and endothelial cells and in developing brain of mouse embryos. Transgenic mice made with a 15kb human EpoR (1.8kb 5' and 7kb 3') showed moderate expression level of the transgene in hematopoietic tissues and sustained low levels of expression in the brain. Now we have made transgenic mice with the two larger (80kb) P1 constructs containing extended upstream and downstream regions from the human EpoR. Analysis showed that the transgene is appropriately expressed in hematopoietic tissues including yolk sac, fetal liver, adult spleen and bone marrow at levels comparable to the endogenous murine EpoR. The 80 kb transgene also provided high level expression in the embryonic brain that paralleled the levels of the endogenous murine EpoR and was no longer detectable after birth. These data suggest that the high level of embryonic brain expression may be relevant to the human EpoR gene and that the transgenic mouse with an 80kb fragment is a suitable model for studying the regulation and possible functional importance of human EpoR expression in the developing embryo. RT-PCR analysis of normal mouse tissues showed that there is an abnormal form of EpoR in brain. When various tissues were screened for endogenous EpoR expression, primers picked from the 3' region of the cDNA of mouse EpoR was able to pick up EpoR signal on brain samples while those picked from the 5' region including exon 1 did not. Under the same condition, both primer pairs detected the EpoR message in spleen and bone marrow. Experiments are in progress to determine whether the phenomenon is due to alternative initiation of transcription, alternative splicing, differential stability of the RNA message or other factors. The answer to this question will provide insight into the biological significance of EpoR expression in the brain.